Field
The instant invention relates to spectrometers configured to reduce interference caused by a material disposed adjacent (directly or indirectly adjacent) and/or proximate (e.g., in close proximity) to a sample of interest.
Background
It is well known that Raman spectroscopy can be performed at angles other than zero degrees. The popular zero degree method, also known as epi-illumination, has advantages with regard to alignment. For example, the method described by Carron et al. in U.S. Pat. No. 7,403,281 entitled “Raman Spectrometer,” which is hereby incorporated by reference in its entirety as if fully set forth herein, describes an epi-illumination scheme in which a system operator does not need to align the laser excitation at the sample with the collection optics and small aperture. This is possible because the manufacturer pre-aligns the beams with a beam splitter.
While this approach is creates an easy to use method for Raman spectroscopy it can also create a problem when an illumination beam of a spectrometer passes through a different material to illuminate a sample. This may create a region of interference 2 as illustrated in FIG. 1. The interference may arise from a container material and/or a material between the container surface and the desired sample. Dr. Carron in his PhD dissertation, Surface Enhanced Resonance Raman, Resonance Hyper-Raman, and Hyper Raman spectroscopy of Molecules Absorbed to Thin Metal Films (1985, Northwestern University), describes a method of providing an excitation signal 3 at an angle 4 to eliminate and/or reduce the region of interference 2 shown in FIG. 1. FIG. 2, taken from Carron's dissertation (page 151), illustrates this method. The approach removed Raman scattering from a window material 6 or solution 8 prior to the sample of interest 10.
FIG. 3 and FIG. 4 illustrate in detail the advantage of the off-axis method used by Carron (1985). In FIG. 3, for example an excitation signal 3 is directed toward a sample 10 at an off-axis angle 4 by a mirror 12. The excitation signal 3 travels through a window material 6 and a solution 8 to the sample 10. FIG. 4 illustrates how a spatial filter 11 of a disperse Raman system may be used to remove interference 12, collectively corresponding to interference 13 from a window 6 and interference 14 from another material, such as the solution 8, disposed prior to the sample 10. The spatial filter 11, in contrast, passes an image/Raman scattering 15 corresponding to the sample 10 to a detector. Raman scattering from other materials (e.g., the window 6 and/or solution 8) or other interference such as fluorescence can be removed by the spatial filter 11.